1. Technical Field
The present invention relates to a heavy-load radial tire which can prevent uneven wear of a tread portion.
2. Related Art
FIG. 8(A) shows a profile of a tread surface (hereinafter, also referred to simply as “tread profile”) “a” of a heavy load radial tire. As shown with a phantom line in FIG. 8(A), if high air pressure is charged into the heavy load radial tire having the tread profile formed from a single arc, there is a tendency that the tread surface a largely swells outward in a radial direction of the tire at a substantially intermediate position j between a tire equatorial plane Co and a tread edge Te. Hence, a tire radius is largely varied between the intermediate position j and the tread edge Te. As a result, uneven wear such as shoulder wear “m” (shown with a phantom line in FIG. 8(C)) in which the tread surface a on the side of the tread edge Te is worn at an early stage is easily generated due to slip with respect to a road surface.
Japanese Patent Application Publication No. 2006-76359 teaches a tread surface in which to suppress the swelling when air pressure is charged, a tread profile at the time of vulcanizing molding is formed from a crown portion a1 of a radius of curvature TR1 having a center on a tire equatorial plane and a shoulder portion a2 which intersects with the crown portion a1 at an intersection point P and which is formed from a straight line extending in an axial direction of a tire, as shown with a phantom line in FIG. 8(B).
when high air pressure is charged into such a tire, the tread profile approaches a single arc, and a difference Δr between a tire radius of the intermediate position j and a tire radius of the tread edge Te becomes smaller than that shown in FIG. 8(A). Hence, slip between the tread surface and the road surface is suppressed from “start of ground contact” to “completion of ground contact” during running. However, on the tread surface in FIG. 8(B), ground contact pressure at the tread edge Te is increased, and the tread edge Te easily slips with respect to the road surface at the moment of the “completion of ground contact”. As a result, as shown in FIG. 8(C), stepped wear n in which only a portion of the shoulder portion a2 in the vicinity of the tread edge Te is worn in a form of a step is easily generated in such a tread surface.
FIG. 9 is a conceptual diagram showing a ground contact state between a road surface and a shoulder portion a2 of a tread portion. Generally, when the difference Δr between the tire radii is small and the ground contact pressure is high, shear deformation is generated in the shoulder portion a2 at the time of the “start of ground contact”, and the motion proceeds toward the “completion of ground contact” while keeping the state where the shear deformation is generated. Since the ground contact pressure is abruptly lowered at the time of the “completion of ground contact”, the sheared and deformed shoulder portion a2 tries to restore to its original shape. This instantaneously generates slip K in the same direction as a tire-rotating direction F between the shoulder portion a2 and the road surface. It is presumed that the stepped wear n is caused by this slip K.
The shoulder wear “m” is caused by the difference Δr between the tire radii, and is generated by the slip with respect to the road surface (slip Kin the same direction as the tire-rotating direction F) which is continuously generated from the “start of ground contact” to the “completion of ground contact”. Hence, the shoulder wear “m” is different from the stepped wear “n” in generation mechanism. The shoulder wear “m” occurs in the form of an inclined surface, and is different from the stepped wear “n” in the wearing shape.